PIE *n̥bʰrós/*n̥bʰrís "rain-cloud, rain, cloud" is probably a zero-grade derivative of *nebʰ-, the root that also gave *nébʰos. However, that derivative remains a bit problematic because beside clearly matching Sanskrit, Avestan, Latin and Armenian cognates there are formal problems with Ancient Greek ὄμβρος (ómbros) with problematic initial ο- and the cluster -μβ- (proper Greek reflex after the expected assimilation of *n to *m to the following labial would be -μφ-), as well as with ἀφρός (aphrós, “foam”), which does formally provide a perfect match, but is usually dropped from consideration due to heavy semantical mismatch (cf. Beekes, Frisk 1960:197 etc.).
- Latin: imber